The present invention relates to a digital input unit with means for detecting wire breakage and with at least one input channel to which an input signal can be supplied over a line; this input signal is transmitted by electrical isolating means of an evaluation unit that assigns the level states of the input signal above a first threshold to a logical xe2x80x9c1xe2x80x9d and the level states between a voltage greater than 0 V, but below a second threshold that is smaller than the first threshold, to a logical xe2x80x9c0xe2x80x9d.
A digital input unit of this type is described in Siemens catalog ST 70, 1997 edition, pages 4/16 and 4/17. To enable electrical isolation, it is customary to provide each input channel with a first and a second optocoupler, one of which is provided to transmit the input signal, i.e., to transmit status information logical xe2x80x9c0xe2x80x9d and logical xe2x80x9c1xe2x80x9d, while the other is provided to transmit wire breakage information to a wire breakage monitoring unit that monitors the lines connected to the input circuit.
An object of the present invention is to simplify the transmission of status and wire breakage information in a digital input unit.
The digital input unit according to the present invention has the advantage that it requires only one optocoupler for transmitting the status and wire breakage information. This significantly improves EMC performance, particularly in digital input units with multiple input channels.